Dissemination of high-end terminals and outspread of wired and wireless networks have given rise to more and more increased opportunities to transmit or view videos via networks. When a video is to be transmitted on a real-time basis or at least with low latency using a limited network bandwidth, the video is usually encoded with compression coding and rate-controlled to be matched with the bandwidth prior to transmission. Intended purposes of such video transmissions include, for example, video conference, video chat, monitoring through security cameras and distribution of live videos (for sports, concerts, etc.).
Conventional compression coding technologies such as MPEG-2 and H.264/AVC provide support of motion compensation based on inter-frame prediction (inter-prediction) in addition to intra-frame prediction (intra-prediction) thereby achieving high coding efficiency. However, in a case where encoded information that would have transmitted becomes unavailable due to any reason such as a packet loss, the inter-frame prediction will not work well and decoding of contents will fail. Upon such a failure, it is possible to recover normal decoding/reproduction of the contents by transmitting at least one image encoded solely with intra-prediction (herein referred to as I (Intra) picture). This type of transmission is called ‘refresh’. However, as an amount of codes of an I picture is generally significantly large compared to the other types of pictures (for example, P pictures or B pictures for which motion compensation is usable), transmission delay or another decoding failure may occur during recovery.
The patent literature 1 discloses a technology to suppress increase in the amount of codes during the above-mentioned refresh by performing the refresh in a distributed manner over a plurality of frames per a partial region basis. The patent literature 2 discloses a technology to dynamically control search area of motion compensation such that no inter-frame reference is made to a region that has not yet recovered in performing the distributed refresh over a plurality of frames.
H.265/HEVC (hereinafter, referred to as HEVC) is a compression coding technology subsequent to H.264/AVC, that was standardized by the Joint Collaboration Team-Video Coding (JCTVC) which is the joint standards group of the ITU-T and the ISO/IEC (see the non-patent literature 1).